1. Field of the Invention
The invention relates to the transformation of sewage sludge, septic tank pumpings, municipal wastewater sludge, manure, and other biological waste (all of which are referred to herein as "wastewater sludge") into fertilizer.
2. Description of the Art
Methods have been developed to produce a fertilizer by adding alkaline materials to wastewater sludge and further processing the resulting material so that substantially all pathogens are killed but the product is not sterilized--i.e., non-pathogenic microorganisms, which would be beneficial to soil, remain alive. Such methods are disclosed in the following United States Patents, the disclosure of each of which is hereby incorporated by reference:
______________________________________ Nicholson 4,554,002 Nov. 19, 1985 Nicholson 4,781,842 Nov. 01, 1988 Nicholson, et al. 4,902,431 Feb. 20, 1990 Burnham 5,135,664 Aug. 04, 1992. ______________________________________
The alkaline materials referred to herein include those described in the incorporated patents.
In order to produce a fertilizer which can be used economically, the foregoing methods must culminate with a sub-process which reduces the liquid content of the moist, partially treated sludge/alkaline mixture (hereinafter referred to as the "mixture") and yields an aggregate which can be easily handled and spread. In current commercial applications of the methods disclosed in these patents, this sub-process comprises windrowing the mixture and periodically turning it over a period of about six or seven days. This is done in a large building in order to protect the mixture from the elements. This sub-process is costly and time consuming.
The disclosures of the foregoing patents suggest the possibility of performing this final sub-process by heated aeration, but they do not provide details of how to do this.
A heated aeration sub-process should satisfy the following criteria:
1. To operate quickly (e.g., to operate on-line and keep up with the prior sub-processes). PA1 2. To establish, maintain and enhance the separation of particles of the partially processed material so that the end product is easily spreadable--e.g., it is granular and has a substantially uniform moisture content. PA1 3. To avoid sterilizing any significant part of the mixture.
The first two criteria are most easily achieved by using high temperatures and drying the product to a very low moisture content, such as, for example, 10%. However, high temperatures and a very low moisture content jeopardize the ability to meet the third criterion of non-sterilization. The sludge/alkaline mixture can be sterilized if held at a high temperature for too long or if dried excessively. Moreover, producing a product which retains enough moisture to maintain microorganisms creates a risk of clumping and clogging and therefore jeopardizes achievement of the first criterion.